The operation of electronic devices such as computers, televisions, telecommunications devices, medical instruments, and the like is attended by the generation of electromagnetic radiation by the electronic circuitry of the devices. Such radiation often develops as an electric field or as transients within the radio frequency band of the electromagnetic spectrum and is generally termed “electromagnetic interference” (“EMI”) and/or “radio frequency interference” (“RFI”). Left un-attenuated, such EMI may interfere with the operation of other nearby electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed. Such shielding may both confine the EMI energy within a source device, and insulate that device from other EMI sources. Such shielding is generally configured as an electrically conductive and grounded housing/enclosure that surrounds the EMI generating circuitry of the source device.
An ideal EMI enclosure would be completely sealed and would provide almost complete EMI shielding. However, most electronic devices and in particular microprocessors generate large amounts of heat energy that must be removed from the enclosure to permit continued operation of the device. Accordingly, most enclosures designed for EMI shielding contain multiple ventilation apertures or other electrically grounded perforated/porous structures (e.g., screens) to permit air flow for cooling purposes.
If properly sized, openings through an electronics enclosure may contain EMI within the enclosure. More specifically, an opening within the enclosure will prevent propagation of electromagnetic waves if the maximum dimension of the opening is less than about one-fourth of the smallest wavelength (highest frequency) to be attenuated. Accordingly, the maximum dimension (e.g., diameter of a circular opening) is inversely related to the operating frequency of the electronic circuitry within the enclosure. For instance, an electronic device operating at 3.0 GHz may result in non-negligible harmonic frequencies as high as 15 GHz. To prevent waveform propagation at such frequencies, openings may be restricted to a maximum allowable dimension of 5 mm or less. Accordingly, as operating frequencies of electronic devices continue to increase, the maximum allowable dimension of ventilation apertures within electronic enclosures continues to decrease.
As the maximum size of ventilation apertures decreases, the impedance of each aperture to air flow increases. This is due in part to boundary effects of the periphery of the ventilation apertures on fluids passing through the aperture. Accordingly, to provide adequate fluid flow for cooling purposes, the surface area of electronics enclosures dedicated to ventilation apertures for cooling electronics has increased with the increase of operating frequencies.
Increasing the surface area on electronics enclosures dedicated to ventilation openings conflicts with other trends in electronics devices, namely, increased power/computing density and reducing the size of electronics enclosures. In many applications (e.g., telecommunications centers, server farms, etc.) it is desirable to increase the number of components (e.g., circuit boards, processors etc.) incorporated into a single electronics enclosure while reducing the overall size of the electronics enclosure. The increased electronic component density of such electronics enclosures may require that more of the surface area of the electronics enclosure be dedicated to electrical connectors, power supplies and the like associated with the electronic components within the enclosure. Accordingly, while the area needed for ventilation apertures on the surface of electronics enclosures is increasing, the area available for such ventilation apertures is decreasing.
It is against this background that the present invention has been developed.